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Microsoft KB Archive/168515

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Knowledge Base

Explanation of an OSPF Area

Article ID: 168515

Article Last Modified on 11/1/2006


  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0

This article was previously published under Q168515


An Autonomous System (AS) in an Open Shortest Path First (OSPF) routed network can be broken up into areas. An OSPF area, based on the same concept as IP subnets, consists of a group of networks logically linked and sharing a single link state database (LSDB).


Routers within a single area maintain the same LSDB and communicate this information to each other by flooding the area with link state advertisements (LSA).

A router that is connected to more than one area is called an area border router (ABR). ABRs are most commonly used to connect tie areas together by being connected its own area and the backbone area. An area border router advertises a summary of its area information to other area border routers using summary advertisements. This area topology significantly reduces the amount of traffic between routers. It also reduces the size of the LSDB within an area. Since the LSDB is used to calculate the SPF, this size reduction can be a significant performance enhancement on the router.

Area ID's are 32-bit numbers and in dotted decimal notation. The area ID is in no way related to the IP address of a network. However, it is possible to logically associate these numbers for administrative convenience.

Additional query words: prodnt 4.00 steelhead rras

Keywords: kbnetwork KB168515